A positive thermal coefficient (PTC) layer is typically employed in a secondary battery, such as a lithium-ion secondary battery, to provide protection against short circuits external to the battery (or cell), i.e., interrupting the current path when an overcurrent or an overvoltage is supplied.
A typical PTC layer includes material(s) having an increasing electrical resistivity with increasing temperature. They generally include small quantities of semiconductor material in a polycrystalline ceramic, or a slice of plastic or polymer with carbon grains embedded in it. When temperature reaches a critical point, the semiconductor material or the plastic with embedded carbon grains forms a barrier to the flow of electricity and resistance climbs very quickly. The temperature at which this occurs can be varied by adjusting the composition of the PTC materials. This on-off behavior of PTC materials is useful in situations where equipment can be damaged by easily definable events, such as batteries, including secondary lithium-ion batteries. For example, when exposed to an overcurrent situation, this normally conductive PTC layer heats up and changes phases to become several orders of magnitude more resistive. Once the short circuits are removed, the PTC layer cools down and returns to its electrically conductive state.
Typically, such PTC layers require their surface area to increase in order to increase the operating current rating of batteries. That is, for high current charging rates in batteries, in particular batteries having a capacity of greater than about 3.0 Ah/cell, PTC layers with relatively large surface areas are generally required. Generally, however, in batteries integrated with PTC layers in the art, the PTC layers have been incorporated within the batteries, thereby significantly limiting their surface area, while occupying the space within the batteries, thereby also reducing the capacity of the batteries.
Therefore, there is a need for a new battery design to accommodate a PTC layer having a relatively large surface area, for example, to maximize the current capability of the PTC layer for a given cell.